Conventionally seam-welding is known, in which a plurality of workpieces to be welded are sandwiched between a pair of electrode rollers, and the workpieces are welded while rotating the electrode rollers, to which pressure and electricity are applied, and while continuously moving the contact point between the electrode rollers and the workpieces using a robot.
Patent Literature 1 describes the technique of detecting the load applied in the direction that is perpendicular to the direction to press the workpiece by the electrode rollers at the welding point and to the delivering direction of the workpiece by the robot at the welding point, and turning the electrode rollers about the pressing direction in accordance with the detected load for track correction.